There are many applications in automobiles requiring the use of power transistors to drive DC motors, solenoids, and other loads exhibiting inductive characteristics. Examples of these applications include electric fuel pumps, anti-skid braking systems, automatic transmission controllers, windshield wipers, power windows, etc. There are also numerous industrial and commercial applications including robotic mechanical controls and small industrial machines.
Power transistors used to drive DC motors and solenoids have included bipolar, Darlington, and more recently Metal Oxide Semiconductor (MOS) field effect transistors. Common circuit configurations include the "H" switch for driving DC motors in forward and reverse directions as described in U.S. Pat. No. 4,454,454, and high side and low side drivers for driving motors in one direction or for powering solenoids. The use of power transistors to drive inductive loads in automobile applications subjects the power transistors to many environmental hazards. These hazards include inadvertently shorting the outputs of the power devices to the ground supply voltage, accidentally reversing the battery terminals while connecting or jumping the battery, loss of a battery terminal connection while the ignition is turned on or the automobile is running, or electrical problems in the wiring or charging system.
Driving inductive loads presents a special problem because when current abruptly stops flowing through the inductive load, a voltage of the opposite polarity and of the same magnitude of the original driving voltage appears at the driving node of the inductive load due to the collapsing electric fields in the windings of the inductor. This condition, known as kick-back, can cause damage to the power transistor driver circuits if proper protection is not provided or if the inductive energy is not properly discharged.
Presently, there are several driver circuits using bipolar, Darlington, and MOS power transistors which can detect a short circuit at the inductive load and provide protection therein. However, if the battery connection is reversed or the battery is disconnected, the present circuits are unable to discharge the resulting negative inductive load voltage properly. In some cases this can permanently damage the driver circuit requiring its replacement in the automobile. In automobile applications it is desirable to provide additional protection in order to have a more fail-safe system.
Thus, what is needed is a circuit for protecting a driver circuit by detecting reverse battery conditions or loss of the positive supply voltage with the ability to quickly discharge the energy of an inductive load.